The oil and gas drilling industry has been undergoing dramatic technology improvements in the last decade, particularly in MWD (Measurement-While-Drilling), directional and horizontal drilling, improved drilling tools and equipment and improved analysis and monitoring capabilities. The combined effect is that drilling cost has been steadily declining, and directional drilling, particularly high-angle, extended reach, and horizontal drilling have become much more popular, and will further see expanded application in the future.
At the same time, due to operators' cost cutting efforts and down-sizing, more and more wells are being drilled on a "turn-key" basis, whereby service companies are asked to contract the entire drilling project at a predetermined benchmark fee, with huge incentives for faster and better drilling, and similar penalties for incurring drilling problems and drilling delays.
The advent of these turn-key projects creates an economic condition under which service companies that are able to improve aspects of the entire drilling operation will reap major profits, and those who do not may suffer major losses. One single severe incident of stuck-pipe can mean a loss of hundreds of thousands of dollars in revenue loss, and possibly more.
As a result, there are important considerations facing the next generation of directional drilling. First, it is important to maximize the length of each bit run. This requires the use of long-life PDC bits, MWD measurements which do not require additional wireline reconfirmation runs, and proper trajectory control. Secondly, it is necessary to minimize the need for course corrections. In rotary drilling, course correction requires tripping, which is very expensive in long reach wells. It also frequently shortens the bit life. In downhole motor drilling, course correction means more crooked borehole paths, resulting in increased torque and drag. The time involved in course correction not only impacts the drilling time, but may also adversely impact hole stability and the formation evaluation process. Thirdly, it is important to improve the initial well path planning. Well paths should be planned to account for the natural deviation and walk tendencies of the drilled well path due to the interaction of the BHA-bit-formation system. Imposing unrealistic well path designs will result in more frequent course corrections or can cause the missing of the target. Currently most wells are planned as 2-D wells. If the natural walk tendency is strong, this will drastically increase the number of course corrections required. Fourthly, smooth well paths are desirable. This requires improved MWD directional surveys, improved trajectory control, by a combination of active trajectory deflection means, and preferably combined with a physically sound simulation of the phenomenon of the intrinsic drilling deviation and walk tendencies due to the interaction of the BHA-bit-formation system. Additionally, it is preferable to maximize the horizontal section of the horizontal well. It is recognized that increasing the horizontal section greatly increases the effective recovery area of the reservoir, reduces the unit drilling cost, and may enable many marginal fields to become economically feasible for development. The major current limitation to the length of the horizontal section is torque and drag. Part of the problem is that the horizontal section is not a straight nor smooth section. It actually consists of a series of alternating and meandering curved sections. This is due to the current practice of using bent housing assemblies and alternating sliding and rotary drilling. To improve the straightness of the horizontal section, it is necessary to reduce the inherent build tendency of the downhole assembly, and to improve the tool face setting operations. For such purpose, a better understanding and control on what really happens to the downhole motor/bent housing assembly is very important. Relying on a sophisticated downhole BHA analysis program is insufficient. Additional downhole measurements are needed to better define the behavior of the assembly.
In directional drilling, especially in long reach, high angle, or horizontal drilling, great emphasis is placed in long bit runs, smooth and properly controlled well paths, and minimal course corrections. Otherwise, major drilling difficulties can develop. In actual drilling, many downhole trajectory control devices are used to deflect the drilling trajectory whenever necessary. These include downhole bent housings of the downhole motor, bent subs or whipstocks, and other active or adjustable devices such as adjustable stabilizers. To properly execute the trajectory deflection, it is very important to set the tool face accurately.
A current method of setting the tool face angle relies on measuring the tool face angle at the location where downhole survey sensors are located in the BHA (bottomhole assembly). However, due to the interference fit caused by such downhole deflection devices, significant contact forces are generated by such devices at the contact points (i.e., the bent knee and the intervening stabilizers). These restraining torques prevent the bent knee from turning when the surface torque is applied. Therefore, the "apparent tool face" at the sensor location very often differs significantly from the true tool face angle at the bent knee.
The prior art method of downhole tool face setting is to infer the tool face at the axial location where the survey sensors are located through survey measurements. The effect of the "restraining torque" at the bent knee and any other intervening contact locations (such as the upper stabilizer of the downhole motor) is not accounted for. As a result, it not only affects the accuracy of the tool face, but also the azimuth accuracy of the directional survey, since the survey data are influenced by the deformation of the downhole assembly. It is accepted that the azimuth accuracy in MWD survey, particularly near the horizontal section, is very poor. Errors of over two degrees in azimuth from such surveys are fairly common. The uncertainty of the well trajectory, due to such azimuthal error, will either lead to strayed drilling or to a crooked horizontal well path. This greatly limits the maximum drillable horizontal extent of the well.
Various U.S. patents have issued to the present inventor in the field of the present invention. U.S. Pat. No. 4,848 144 (issued on Jul. 18, 1989), U.S. Pat. No. 4,972,703 (issued on Nov. 27, 1990), and U.S. Pat. No. 5,044,198 (issued on Sep. 3, 1991) have addressed methods of predicting the torque and drag in directional wells. These patents describe a method for generating an improved torque-drag model for at least the collar portion of the drillstring in a directional oil or gas well. The techniques of these patents determine the stiffness of incremental portions of the drillstring, and uses this information, along with the borehole clearance and the borehole trajectory, to determine the contact locations between the drillstring and the sidewalls of the well. The contact force at these determined locations can be calculated, taking into consideration all significant kinematic, external, and internal forces acting on that incremental portion of the drillstring. More accurate torque-drag analysis, provided by the model of these patents, assists in well planning, prediction and control, and assists in avoiding drilling problems. This method serves to reduce total costs for the well.
It is an object of the present invention to provide a method for accurately setting the tool face angle.
It is another object of the present invention to provide a method that can more effectively control the straightness of the horizontal or directional drilling borehole.
It is a further object of the present invention to provide a method that can provide greater information concerning the borehole path profile.
It is still a further object of the present invention to provide a method which can minimize the problems of directional drilling.
These and other objects and advantages of the present invention will become apparent from a reading of the attached specification and appended claims.